1. Field of the Invention
This invention relates in general to a vehicle speed control apparatus and in particular to an automatic speed control circuit which adjusts the vehicle throttle in response to a comparison of the vehicle road speed with a vehicle operator selected desired road speed.
2. Description of the Prior Art
Early devices for controlling the speed of a vehicle were mechanical controls having a large number of interrelated moving parts. These devices were usually complex and therefore expensive to manufacture and difficult to maintain in an accurate operating condition. Furthermore, since they were linked to the vehicle throttle, there were normally positioned in the engine compartment proximate the throttle linkage. This positioning posed problems where they were driven from the speedometer cable which then had to be brought into the engine compartment before being attached to the speedometer head in the instrument panel.
The problems associated with the mechanical speed control devices were overcome by the use of an electrical control device. At a comparable cost, the electrical device could be made more accurate and more reliable than the mechanical device. Furthermore, it could also be positioned proximate the throttle linkage without disturbing the speedometer cable since it only required an electrical connection to the speedometer head or cable to pick up the vehicle road speed. However, the electrical control had some associated problems.
If the reference road speed and/or the actual vehicle road speed signals were generated by the charging of a capacitor to a voltage potential proportional to the speed, inaccuracies were introduced due to leakage or deterioration over long periods of time. Furthermore, a relatively large capacitor was required to store a sufficient amount of charge and therefore the system package was relatively large. Another problem involved the use of a dial type control for setting in the reference road speed. These controls presented a safety hazard since the driver's attention was diverted from the road.
U.S. Pat. No. 3,715,006, issued Feb. 6, 1973 to William J. Walsh et al., discloses an automatic electrical speed control apparatus which eliminates the above mentioned capacitor and dial type control. The apparatus includes an A.C. source for generating a signal having a frequency proportional to the vehicle road speed. A frequency to voltage converter converts the speed signal to a D.C. voltage road speed signal having an amplitude proportional to the instantaneous road speed. A free-running pulse source for supplying pulses of a fixed amplitude and frequency is connected through a gate to a counter which accumulates the pulses in a memory. The count is converted to a D.C. voltage count signal having an amplitude proportional to the number of pulses counted. The gate is opened when a set switch is actuated by the operator and is closed when the D.C. road speed signal is equal to the D.C. count signal as detected by a comparator. The total count therefore is proportional to the reference road speed.
The D.C. road speed signal and the D.C. count signal are also the inputs to a differential amplifier which generates an output signal which varies with the changes in the relationship between the D.C. road speed signal and the D.C. count signal. A pulse width modulator responds to the differential amplifier output signal by generating a constant frequency signal having a duty cycle proportional to the output signal. The modulated signal is utilized to control a valve driver which in turn drives a solenoid connected to the throttle linkage to adjust the vehicle road speed. A low speed inhibit circuit prevents actuation of the solenoid below a minimum speed, typically 25 miles per hour.
The Walsh et al. speed control apparatus utilizes a resistor-capacitor filter in its frequency to voltage converter which either may react too slowly to changes in the road speed or may pass a ripple voltage at the tach generator frequency to cause an inaccurate reference road speed signal. Furthermore, since the solenoid of this speed control apparatus is not actuated until the set switch is released, the speed control apparatus may allow the vehicle to lose speed until the solenoid "catches up".